The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus, a device for transferring a color image from an intermediate transfer body to a recording medium, and a method of conveying a recording medium to a transfer region where a color image is to be transferred from an image carrier to the recording medium.
One of conventional image forming apparatuses includes an image carrier and an elastic transfer member contacting the image carrier and forming a nip or transfer region between it and the image carrier. A bias for image transfer is applied to the nip in order to transfer a color image electrostatically carried on the image carrier to a paper sheet or similar recording medium. A gap exists between the image carrier and the transfer member at each of the upstream side and downstream side in a direction in which the recording medium is conveyed (direction of conveyance hereinafter). The bias for image transfer forms electric fields in such gaps also.
Assume that a paper sheet is present in the gap at the upstream side in the direction of transfer and spaced from the image carrier. Then, the electric field formed in the gap causes a color image formed on the image carrier to fly toward the paper sheet, resulting in so-called pretransfer. Let the region where the pretransfer occurs be referred to as a pretransfer region. Toner is scattered around an expected image as a result of the pretransfer. This problem arises not only in an image forming apparatus of the type pressing a paper sheet against an image carrier from the side of the paper sheet opposite to the image transfer side, but also in an image forming apparatus of the type affecting image transfer by causing the image transfer side of a paper sheet to contact an image carrier. The latter type of image forming apparatus may be one in which a transfer charger charges a paper sheet from the side of the paper sheet opposite to the image transfer side in order to form an electric field for image transfer.
It is a common practice with a full-color copier or similar full-color image forming apparatus to transfer a toner image or color image from a photoconductive element to an intermediate transfer body (primary transfer) and then to a paper sheet being conveyed in close contact with the intermediate transfer body (secondary transfer). In this case, the scattering of toner ascribable to the pretransfer blurs the toner image on the paper sheet.
Presumably, a paper sheet parts from the image carrier in the pretransfer region due to the following two different causes. One causes relates to the curvature of the image carrier in the transfer region while the other cause relates to a manner in which guide members guide a paper sheet toward the nip, as will be described specifically later with reference to the accompanying drawings.
Japanese Patent Laid-Open Publication No. 6-3974 discloses an image forming apparatus including a countermeasure against the scattering of toner to occur in the pretransfer region. The image forming apparatus includes a transfer member for electrostatically transferring a toner image from an image carrier to a paper sheet. The transfer member and image carrier form a nip therebetween. An upper or first guide member and a lower or second guide member cooperate to guide a paper sheet to a transfer position. The ends of the upper and lower guide members positioned at the most downstream side in the direction of conveyance are located on or above a line tangential to the image carrier at the most upstream point of the nip. Further, the angle between the guide surface of each of the two guide members and the horizontal is selected to be smaller than the angle between the above tangential line and the horizontal.
The upper and lower guide members, satisfying the above-described conditions, allow a paper sheet to enter the nip in contact with the image carrier. The document teaches that a gap causative of the pretransfer does not exist between the image carrier and the paper sheet in the pretransfer region. The paper sheet, in accordance with the document, contacts the image carrier and then enters the nip, i.e., it does not directly enter the nip. However, the point of the image carrier that the leading edge of the paper sheet contacts is dependent on the degree of flexibility of the paper sheet. If the distance between the above point of the image carrier and the inlet of the nip is excessively great, the paper sheet noticeably bends due to a difference in conveying speed between a registration roller pair and the transfer member. The bend of the paper sheet is apt to occur in the pretransfer region. It is therefore likely that the paper sheet parts from an intermediate transfer belt in the pretransfer region.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-46031, 5-61365, 5-341670, 10-39648 and 2000-75676.
It is therefore an object of the present invention to provide an image forming apparatus, an image transferring device and a recording medium conveying method capable of obviating pretransfer by maintaining a recording medium in close contact with an image carrier in a pretransfer region and thereby insuring attractive images free from the scattering of toner.
In accordance with the present invention, an image forming apparatus includes an image carrier for carrying a color image thereon. An elastic transfer member contacts the image carrier to thereby form a nip for electrostatically transferring the color image from the image carrier to a recording medium, and causes the recording medium being conveyed toward the nip to contact the image carrier and then enter the nip. A first guide member guides one side of the recording medium expected to receive the color image to the nip. The first guide includes, in a portion thereof contacting the recording medium other than the leading and trailing edges of the medium in the direction of conveyance while guiding the medium, a first restriction point located at the most downstream side in the above direction. A second guide member guides the other side of the recording medium to the nip. The second guide member includes, in a portion thereof contacting the recording medium other than the leading and trailing edges of the medium in the direction of conveyance while guiding the medium, a second restriction point located at the most downstream side in the direction of conveyance and downstream of the first restriction point. The first and second guide members are positioned such that the first restriction point is positioned at the opposite side to the image carrier with respect to a reference line connecting the second restriction point and the upstream end of the nip in the direction of conveyance.
Also, in accordance with the present invention, in a method of conveying a recording medium, to which a color image is to be electrostatically transferred from an image carrier, to a nip for image transfer formed between the image carrier and an elastic transfer member such that the medium contacts the image carrier and then enters the nip, the medium is conveyed while being restricted such that the most downstream point of a restricting portion, which restricts the image transfer side of the medium, other than opposite ends in the direction of conveyance is positioned at the opposite side to the image carrier with respect to a reference line connecting the most downstream point of a restricting portion, which restricts the other side of the medium, other than opposite ends in the direction of conveyance and the upstream end of the nip in the above direction.
Further, in accordance with the present invention, an image forming apparatus includes an intermediate transfer body for carrying a color image thereon. A transferring device conveys a recording medium while causing it to bend and move along a part of the intermediate transfer body positioned at the upstream side in a direction of conveyance in a transfer region, in which a color image formed on the intermediate transfer body is transferred to the medium. A contact assisting member is positioned upstream of the transfer region in the direction of conveyance for maintaining the recording medium and intermediate transfer body in close contact with each other.
Moreover, in accordance with the present invention, in an image transferring device for transferring a color image formed on an intermediate transfer body to a recording medium being conveyed by being warped such that the medium moves along part of the intermediate transfer body positioned upstream of a transfer region in a direction of conveyance, a contact assisting member is positioned upstream of the transfer region in the above direction and configured to maintain the medium and intermediate transfer body in close contact with each other.